The present invention relates generally to ion beam transport and mass analysis in the presence of a magnetic field. More particularly, the invention relates to an apparatus for selecting ion beams in ion implantation systems using high perveance beams.
Ion implantation has been widely used as one of the major process steps for fabricating integrated circuits. Ion implantation is a process used to selectively implant dopants (elemental or molecular species) into various substrate materials such as semiconductors and metals to obtain useful physical properties and devices. As integrated circuits are miniaturized, ion implantation systems with relatively low energy ( less than 5 KeV) and high current ( greater than 5 mA) are now in demand in order to accommodate specifications of the integrated circuits. However, low energy and high current ion beam transport suffers from effects of space-charge induced divergence (or xe2x80x9cblow-upxe2x80x9d). As a result, ion beam current is excessively lost to apertures and walls of vacuum chambers of the ion implantation system.
Low energy and high current ion beams are generally known as high perveance beams. The perveance is expressed as follows:
P=I(M/m6)xc2xdUxe2x88x923/2 
where P is the perveance, I is the ion beam current, M is the ion mass, me is the mass of the electron, and U is the beam acceleration potential. To effectively transport the high perveance beam, it is necessary to use magnets with a large acceptance. Magnet acceptance is a function of the product of beam cross section and beam envelope angle. It is known that the acceptance of conventional sector magnets may be significantly limited by fringe magnetic fields near the inner and outer arcuate edges of the magnets. The traditional sector magnet optimization can be found in H. F. Glavish, Magnet Optics For Beam Transport, Nuclear Instruments and Methods, 43-55 (1981).
There are several attempts to improve magnet acceptance in the prior art. In xe2x80x9cA Separate Function Magnet Lattice for a Very High Energy Synchrotron,xe2x80x9d Proceeding of International Conference on Magnet Technology, 2nd, edited by H. Hadley, 768, Oxford, England,(1967), Danby et al. use pole rotations and curvature of the magnet pole at the entrance and exit positions. Window frame magnets are also used extensively to minimize fringe field effects.
The space-charge divergence inherent in low energy and high current beams can also be offset somewhat by magnetic focusing through optimizing the system""s beam optics and increasing the magnet acceptance. Contoured yokes are used to modify the distribution of the magnetic field to provide magnetic focusing in particle accelerator systems (cf. Livingston et al., Particle Accelerators, McGraw Hill Book Company, 246-56 (1962)). As disclosed in U.S. Pat. No. 4,740,758 to Ries (1988), corrector coils are also used to provide dynamic adjustment to the magnetic field in high energy particle accelerators to obtain desired magnetic focusing. Such magnetic focusing in accelerator systems involves the introduction of higher order magnetic moments. typically quadruple and sextuple components. into the field region. Analogous approaches have also been used to compensate higher order moments to obtain highly homogeneous dipole fields for use in nuclear magnetic resonance systems as depicted in U.S. Pat. No. 4,509,030 to Vennilyea (1985). Dynamic focusing of low energy ion beams with focusing magnetic quadrupole fields generated by a plurality of quadrupole coils has recently been disclosed in U.S. Pat. No. 5,736,743 to Benveniste (1996) in the field of ion implantation.
Despite all these efforts on trying to deliver a reliable production-worthy low energy and high current implantation system to market, manufacturers have thus far achieved only modest success, usually at the cost of sacrificing beam quality (e.g., high contamination and poor reliability) for beam quantity (e.g., ion beam current).
The invention disclosed herein provides a significant improvement and simplification of the prior art by combining dipole magnet coils with contour yokes and secondary magnet coils to generate a gradient in the dipole field that enhances beam transport. The invention is capable of achieving superior beam currents without sacrificing beam quality while simultaneously reducing the complexity and the cost of production and operation.
The magnet system used in an ion beam implantation system according to the invention comprises a ferromagnetic assembly that defines a magnetic field region therein and has an entrance and an exit for an ion beam that passes through the magnetic field region. At least one primary magnet coil is mounted inside the assembly for generating a first magnetic field in a first direction at the magnetic field region. At least one secondary magnet coil is mounted inside the assembly for generating a second magnetic field in a second direction at the magnetic field region such that a resulting magnetic field has a gradient along one direction at the magnetic field region that enhances ion beam transport.
The assembly has upper and lower mirror-image yokes. Each yoke has a first inner sidewall, a second inner sidewall opposed to and outward of the first inner sidewall, and an inwardly facing contour pole face located between the first and second inner sidewalls. The pole faces of the upper and lower yokes oppose each other. First and second primary magnet coils are mounted on the inner sidewalls of the upper and lower yokes, respectively, such that the direction of the magnetic field generated by these coils is across, preferably perpendicular to, the two pole faces of the upper and lower yokes. First and second secondary magnet coils are mounted in the upper and lower yokes with their respective first portions on the pole faces of the upper and lower yokes. The magnetic field generated by the first portions of the secondary magnet coils is across the first and second inner sidewalls. The first and second secondary magnet coils have second portions that are mounted on the first and second primary magnet coils. As a result, the resulting magnetic field has a gradient along the direction of the first and second inner sidewalls.
Each pole face has a convex shape protruding towards the magnetic field region. Preferably, the convex shape has a groove at the center thereof and the first and second secondary magnet coils have their respective first portions mounted in such grooves. In one embodiment, the first and second secondary magnet coils have their respective second portions mounted on the surface of the portions the first and second primary magnet coils located on the second inner sidewalls.
In a second embodiment of the invention, the magnet system has the configuration as that in the first embodiment except that the first and second secondary magnet coils have their respective second portions mounted on the surface of the portions of the first and second primary magnet coils located on the first inner sidewalls.
In a third embodiment of the magnet system of the invention, four secondary magnet coils are used and each pole face has two grooves adjacent to each other. Each of the secondary magnet coils has a first portion mounted in one of the grooves and a second portion mounted on the surface of the portion of the primary magnet coil located on the nearer inner sidewalls.
The yokes are made of ferromagnetic materials. The pole face may comprise interchangeable pieces for adjusting the shape and strength of the magnetic field in the magnetic field region.